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Configuration Guide - IP Service

CloudEngine 8800, 7800, 6800, and 5800 V200R002C50

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
ARP Implementation

ARP Implementation

Format of ARP Packets

Figure 2-1 shows the format of an ARP Request or Reply packet.

Figure 2-1  Format of an ARP Request or Reply packet

Description of the main fields is as follows:

  • Hardware Type: indicates the hardware address type. For an Ethernet, the value of this field is 1.
  • Protocol Type: indicates the type of the protocol address to be mapped. For an IP address, the value of this field is 0x0800.
  • Hardware Length: indicates the hardware address length. For an ARP Request or Reply packet, the value of this field is 6.
  • Protocol Length: indicates the protocol address length. For an ARP Request or Reply packet, the value of this field is 4.
  • OP: indicates the operation type. The value 1 indicates ARP requesting, and the value 2 indicates ARP replying.
  • Ethernet Address of sender: indicates the MAC address of the sender.
  • IP Address of sender: indicates the IP address of the sender.
  • Ethernet Address of destination: indicates the MAC address of the receiver.
  • IP Address of destination: indicates the IP address of the receiver.

Address Resolution Process

ARP completes address resolution through two processes: ARP request process and ARP reply process.

Figure 2-2  ARP request process

As shown in Figure 2-2, HOSTA and HOSTB are on the same network segment. HOSTA needs to send IP packets to HOSTB.

HOSTA searches the local ARP table for the ARP entry corresponding to HOSTB. If the corresponding ARP entry is found, HOSTA encapsulates the IP packets into Ethernet frames and forwards them to HOSTB based on its MAC address.

If the corresponding APR entry is not found, HOSTA caches the IP packets and broadcasts an ARP Request packet. In the ARP Request packet, the IP address and MAC address of the sender are the IP address and MAC address of HOSTA. The destination IP address is the IP address of HOSTB, and the destination MAC address contains all 0s. All hosts on the same network segment can receive the ARP Request packet, but only HOSTB processes the packet.

Figure 2-3  ARP reply process

HOSTB compares its IP address with the destination IP address in the ARP Request packet. If HOSTB finds that its IP address is the same as the destination IP address, HOSTB adds the IP address and MAC address of the sender (HOSTA) to the local ARP table. Then HOSTB unicasts an ARP Reply packet, which contains its MAC address, to HOSTA, as shown in Figure 2-3.

After receiving the ARP Reply packet, HOSTA adds HOSTB's MAC address into the local ARP table. Meanwhile, HOSTA encapsulates the IP packets and forwards them to HOSTB.

ARP Aging Mechanism

  • ARP cache (ARP table)

    If HOSTA broadcasts an ARP Request packet every time it communicates with HOSTB, the communication traffic on the network will increase. Furthermore, all hosts on the network have to receive and process the ARP Request packet, which decreases network efficiency.

    To solve the preceding problems, each host maintains an ARP cache, which is the key to efficient operation of ARP. This cache contains the recent mapping from IP addresses to MAC addresses.

    Before sending IP packets, a host searches the cache for the MAC address corresponding to the destination IP address. If the cache contains the MAC address, the host does not send an ARP Request packet but directly sends the IP packets to the destination MAC address. If the cache does not contain the MAC address, the host broadcasts an ARP Request packet on the network.

  • Aging time of dynamic ARP entries

    After HOSTA receives the ARP Reply packet from HOSTB, HOSTA adds the mapping between the IP address and the MAC address of HOSTB to the ARP cache. However, if a fault occurs on HOSTB or the network adapter of HOSTB is replaced but HOSTA is not notified, HOSTA still sends IP packets to HOSTB. This fault occurs because the APR entry of HOSTB in the ARP cache on HOSTA is not updated.

    To reduce address resolution errors, a timer is set for each ARP entry in an ARP cache. When a dynamic ARP entry expires, the device sends ARP aging probe packets to the corresponding host. If the host does not respond, the ARP entry is deleted, otherwise, the ARP entry is saved.

    Configuring the timer reduces address resolution errors but does not eliminate the problem because of the time delay. Specifically, if the length of a dynamic APR entry timer is N seconds, the sender can detect the fault on the receiver after N seconds. During the N seconds, the cache on the sender is not updated.

  • Number of probes for aging dynamic ARP entries

    Besides setting a timer for dynamic ARP entries, you can set the number of probes for aging dynamic ARP entries to reduce address resolution errors. Before aging a dynamic ARP entry, a host sends ARP aging probe packets. If the host receives no ARP Reply packet after the number of probes reaches the maximum number, the ARP entry is deleted.

  • Aging probe modes for dynamic ARP entries

    Before a dynamic ARP entry on a device is aged out, the device sends ARP aging probe packets to other devices on the same network segment. An ARP aging probe packet can be a unicast or broadcast packet. By default, a device sends the last ARP aging probe message in broadcast mode, and the rest ARP aging probe messages are sent in unicast mode.

    If the IP address of the peer device remains the same but the MAC address changes frequently, it is recommended that you configure ARP aging probe packets to be broadcast.

    If the MAC address of the peer device remains the same, the network bandwidth is insufficient, and the aging time of ARP entries is short, it is recommended that you configure ARP aging probe packets to be unicast.

    When a non-Huawei device connected to a Huawei device receives an ARP aging probe packet whose destination MAC address is a broadcast address, the non-Huawei device checks the ARP table. If the mapping between the IP address and the MAC address of the Huawei device exists in the ARP table, the non-Huawei device drops the ARP aging probe packet. The Huawei device cannot receive a response and therefore deletes the corresponding ARP entry. As a result, traffic from the network cannot be forwarded. In this scenario, the Huawei device needs to send ARP aging probe packets in unicast mode and the non-Huawei device needs to respond to the ARP aging probe packets.

Dynamic ARP

Dynamic ARP entries are generated and maintained dynamically by using ARP packets. They can be aged out, updated, or overwritten by static ARP entries. When the aging time expires or the interface is Down, the corresponding dynamic ARP entries are deleted.

NOTE:
When the egress of ARP entries is a tunnel, in the event of a tunnel status change, the device does not get aware of ARP entries, and ARP entries are automatically deleted when aged out.

Static ARP

Static ARP entries record fixed mapping between IP addresses and MAC addresses and are configured manually by network administrators.

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Updated: 2019-03-21

Document ID: EDOC1000166635

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